EPDM rubber is a widely used form of synthetic rubber. EPDM rubber is created by the copolymerization of ethylene and propylene with the addition of a diene monomer. Adding a diene monomer, or other suitable unsaturated group, allows EPDM to be vulcanized with sulfur. Due to EPDM's absence of double carbon bonds, EPDM rubber exhibits tremendous resistance to aging and degradative agents such as hot and cold temperatures, oxygen, ozone, acid-base exposure and photooxidation. Due to these resilient properties, EPDM rubber is ideal for use in athletic surfaces, tires, seals, electrical and construction equipment.
A key ingredient in the formula of EPDM rubber is the filler compound carbon black. Carbon black is not an inert filler compound but instead enhances and reinforces various properties of EPDM rubber. While carbon black gives EPDM rubber many of its durable properties, carbon black also gives EPDM its black coloration. However, for commercial reasons, EPDM rubber is often required to be in colors other than black. Unfortunately, methods of coloring black EPDM rubber have proven to be difficult, time consuming and expensive.
For example, in fabricating athletic surfaces such as playground safety surfaces, carbon black EPDM is utilized due to its flexibility and weather resistant properties. However, for aesthetic reasons, there is a commercial demand for athletic surfaces in colors other than black. This requires coloring the EPDM rubber prior to fabricating the athletic surface. Due to the fact that known methods of coloring EPDM rubber are difficult and time consuming, colored athletic surfaces using EPDM rubber are much more expensive than black EPDM rubber surfaces.
For example, one common method of coloring EPDM rubber has been to substitute coloring fillers for carbon black such as calcium carbonate, clay and silicas. However, EPDM rubber compounded with fillers other than carbon black is rigid and less resilient to environmental degradation than carbon black EPDM. For example, non-carbon black EPDM rubber cannot be extruded into layers thinner than one millimeter. This compromises the flexibility and weather resistant properties of the EPDM rubber. Thus, coloration of EPDM rubber with replacement fillers other than carbon black has proven to be undesirable in that many of the advantageous properties of carbon black EPDM rubber are lost.
Another method of coloring carbon black EPDM rubber, described in U.S. Pat. No. 3,935,329, involves halogenating and painting the outer surface of EPDM rubber. However, halogenating the outer surface of EPDM causes delamination of the surface and subsequent loss of flexibility. As a result, the painted outer surface quickly deteriorates.
U.S. Pat. No. 4,442,167 ('167 patent) describes a method of coloring EPDM rubber wherein a thin layer of graft copolymer is added to the outer surface of carbon black EPDM rubber by extrusion molding. The additional layer of colored rubber polymer consists of non-carbon black EPDM rubber and vinyl chloride with coloring filler. As an alternative to combining the layers by extrusion molding, the '167 patent teaches the use of a foaming agent to bind the colored graft copolymer layer to the carbon black EPDM after vulcanization of the rubber. However, both methods are expensive and labor intensive. In addition, adding an outer layer of graft copolymer to an EPDM rubber surface requires the use of a dedicated extruder.
U.S. Pat. No. 5,135,797 describes a method of coloring EPDM rubber using a decorative film fused to the outer surface of the EPDM rubber. The decorative film consists of a substrate layer and a decorative layer which are bonded together by a third adhesive layer. The substrate layer is comprised of an ethylene/acrylic acid copolymer or acrylate copolymer. The decorative layer is formed from an ionomer or fluorine resin or a polyurethane film. However, the above method is flawed in that each layer must be added separately to the outer surface of the rubber substrate which is an extremely time consuming and tedious process.
Another method of coloring EPDM rubber is to simply paint the outer surface of the rubber by conventional methods. However, this results in an outer layer of paint that is rigid and incompatible with the underlying flexible rubber surface. As a result, the outer paint surface cracks and creases and quickly wears off.
The above methods of coloring EPDM rubber are inadequate for fabricating rubber products such as athletic surfaces in that they are either too expensive, too time consuming or simply ineffective. Consequently, there is a need for a cost effective and efficient method of coloring large quantities of EPDM rubber that leaves a permanent and viable coating or color on the outer surface of the rubber.